1. Field of the Invention
The present invention relates to electrically heated furnaces, and more particularly to electrically heated furnaces in which the heating within the furnace takes place as a result of an electric current that flows into a wall of the furnace.
2. Description of the Related Art
In furnace operations, high demands are often placed on the insulation of the heated volume. High demands are also placed on the requirement of uniform temperature distribution within the furnace in respect of different applications. In other words, the greatest acceptable temperature difference throughout the heated volume is often very low. In other applications, it is desired to check and control temperature distribution to a very high degree of accuracy in accordance with a predefined distribution.
Examples of such applications are furnaces for single crystal growth, diffusion furnaces and tube-like furnaces where electric current through the tube wall generates the thermal energy that heats the enclosed volume of the furnace. This heating of the furnace volume requires a high amperage input, which means that the devices through which electric current is taken into and out of the furnace must have a large cross-sectional surface area. The furnace may be a continuous conveyor furnace having open ends, or a furnace that fully encloses the furnace volume.
Tube-like furnace may consist of a tube to which current is supplied. The tube may include an internal ceramic lining. The tube may also be a process tube situated within a surrounding heating coil.
When a temperature gradient exists between the furnace and its surroundings, all devices that are in direct contact with the furnace surface will lead thermal energy away from the furnace to the colder surroundings. This thermal energy drain takes place from the point at which the device concerned is in contact with the furnace surface and is more effective the better the device conducts heat and the larger the contact surface is between said device and the furnace.
Examples of such devices include supports for holding the furnace in place, different measuring devices, and current outlets for supplying current to the furnace surface or leading current away from said surface. These devices are often made of metal and are therefore good heat conductors. When the device in question is a current input device, large electrical contact surfaces are often required due to the strong current required to heat the furnace to the desired temperature.
Typical working conditions for a given type of electrically-heated tube-like furnace include temperatures of from 500-1200° C. inclusive. At these temperatures, a typical highest acceptable deviation from the predetermined temperature distribution in the furnace is 10-20° C. When heating material for single crystal growth by diffusion, the temperature range may be 500-1400° C. with an accuracy of +/−0.1° C. The electric currents required to achieve such working temperatures are so strong as to require the use of relatively powerful current input devices.
Other types of furnaces may be heated in ways other than by supplying electrical energy to the furnace casing. Furthermore, different devices that do not normally conduct current may be applied to the furnace casing and thereby cause the punctiform flow of thermal energy from the heated furnace volume.